Galvannealed steel sheet

Abstract

A method for the manufacture of a galvannealed steel sheet including the provision of a specific steel sheet, a recrystallization annealing with specific heating, soaking and cooling sub-steps using an inert gas, a hot-dip galvanizing and an alloying treatment; the galvannealed steel sheet and the use of the galvannealed steel sheet.

Claims

1. A method for manufacturing a galvannealed steel sheet, the method comprising: A. providing a steel sheet having the following chemical composition in weight percent: 0.05C0.20%, 1.5Mn3.0%, 0.10Si0.45%, 0.10Cr0.60%, Al0.20%, V<0.005%, 0P<0.04%, 0Nb0.05%, 0B0.003%, 0Mo0.020%, 0Ni0.1%, 0Ti0.06%, 0S0.01%, 0Cu0.1%, 0Co0.1%, 0N0.01%, a remainder of the composition being made of iron and inevitable impurities resulting from processing; B. recrystallization annealing the steel sheet in a full radiant tube furnace comprising a heating section, a soaking section, and a cooling section, and optionally an equalizing section, the recrystallization annealing including the following sub-steps: i. heating the steel sheet from ambient temperature to a temperature T1 between 700 and 900 C. in the heating section having an atmosphere A1 including from 0.1 to 15% by volume of H.sub.2 and an inert gas with a dew point DP1 between 18 C. and +8 C., ii. soaking the steel sheet from T1 to a temperature T2 between 700 and 900 C. in the soaking section having an atmosphere A2 identical to A1 with a dew point DP2 equal to DP1, and iii. cooling the steel sheet from T2 to T3 between 400 and 700 C. in the cooling section having an atmosphere A3 including from 1 to 30% H.sub.2 by volume and an inert gas with a dew point DP3 below or equal to 30 C., and iv. optionally, equalizing the steel sheet from a temperature T3 to a temperature T4 between 400 and 700 C. in the equalizing section having an atmosphere A4 including from 1 to 30% H.sub.2 by volume and an inert gas with a dew point DP4 below or equal to 30 C., C. hot-dip galvanizing of the annealed steel sheet in a zinc bath; and D. alloying the galvanized annealed steel sheet, the alloying performed at a temperature T5 between 460 and 600 C. during a time t5 between 1 and 45 seconds.

2. The method as recited in claim 1 wherein in step A), the steel sheet includes less than 0.30% by weight of Si.

3. The method as recited in claim 1 wherein in step A), the steel sheet includes above 0.0001% by weight of V.

4. The method as recited in claim 1 wherein in steps B.i) and B.ii), A1 includes between 1 and 10% by volume of H.sub.2.

5. The method as recited in claim 1 wherein in steps B.i) and B.ii), DP1 is between 15 C. and +5 C.

6. The method as recited in claim 1 wherein in step B.ii), T2 is equal to T1.

7. The method as recited in claim 1 wherein in steps B.i) and B.ii), T1 and T2 are between 750 and 850 C.

8. The method as recited in claim 1 wherein in steps B.iii) and the optional sub-step B.iv), A3 is identical to A4, DP4 being equal to DP3.

9. The method as recited in claim 1 wherein in steps B.i), B.ii) and B.iii) and the optional sub-step B.iv), the inert gas is chosen from the group consisting of: N.sub.2, Ar, He and Xe.

10. The method as recited in claim 1 wherein the zinc-based coating includes between 0.01 and 0.4% by weight of Al, the balance being Zn.

11. The method as recited in claim 1 wherein in step D), T5 is between 470 and 570 C.

12. The method as recited in claim 1 wherein in step D), t5 is between 1 and 35 seconds.

13. The method as recited in claim 1 wherein the chemical composition of the steel does not include Bismuth (Bi).

14. The method as recited in claim 1 wherein the equalizing sub-step is performed.

15. The method as recited in claim 1, wherein the galvannealed steel sheet has a microstructure comprising bainite, martensite and ferrite.

16. The method as recited in claim 15, wherein the galvannealed steel sheet has a microstructure comprising from 1 to 45% of martensite, from 1 to 60% of bainite, the balance being austenite.

17. The method as recited in claim 1, wherein wherein the galvannealed steel sheet has a microstructure comprising from 1 to 25% of fresh martensite, from 1 to 10% of ferrite, from 35 to 95% of martensite and lower bainite and less than 10% of austenite.

18. The method as recited in claim 1, wherein the steel sheet comprises a Dual Phase steel having a ferritic-martensitic microstructure.

19. A method for manufacturing part of an automotive vehicle comprising the method for manufacturing a galvannealed steel sheet as recited in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figure:

(2) FIG. 1 illustrates one method of the prior art disclosed in the patent application JP2011153367.

(3) FIG. 2 illustrates one example of the method according to the present invention.

DETAILED DESCRIPTION

(4) The following terms will be defined: vol. % means the percentage by volume, wt. % means the percentage by weight.

(5) The invention relates to a method for the manufacture of a galvannealed steel sheet comprising: A. The provision of a steel sheet having the following chemical composition in weight percent: 0.05C0.20%, 1.5Mn3.0%, 0.10Si0.45%, 0.10Cr0.60%, Al0.20%, V<0.005% and on a purely optional basis, one or more elements such as P<0.04%, Nb0.05%, B0.003%, Mo0.20%, Ni0.1%, Ti0.06%, S0.01% Cu0.1%, Co0.1%, N0.01%,

(6) the remainder of the composition being made of iron and inevitable impurities resulting from the elaboration, B. The recrystallization annealing of said steel sheet in a full radiant tube furnace comprising a heating section, a soaking section, a cooling section, optionally an equalizing section comprising the sub-following steps: i. the heating of said steel sheet from ambient temperature to a temperature T1 between 700 and 900 C. in the heating section having an atmosphere A1 comprising from 0.1 to 15% by volume of H.sub.2 and an inert gas whose a dew point DP1 is between 18 C. and +8 C., ii. the soaking of the steel sheet from T1 to a temperature T2 between 700 and 900 C. in the soaking section having an atmosphere A2 identical to A1 with a dew point DP2 equal to DP1, iii. the cooling of the steel sheet from T2 to T3 between 400 and 700 C. in the cooling section having an atmosphere A3 comprising from 1 to 30% Hz by volume and an inert gas whose a dew point DP3 is below or equal to 30 C., iv. optionally, the equalizing of the steel sheet from a temperature T3 to a temperature T4 between 400 and 700 C. in the equalizing section having an atmosphere A4 comprising from 1 to 30% Hz by volume and an inert gas whose a dew point DP4 is below or equal to 30 C., C. The hot-dip galvanizing of the annealed steel sheet in a zinc bath and D. An alloying treatment performed at a temperature T5 between 460 and 600 C. during a time t5 between 1 and 45 seconds.

(7) Without willing to be bound by any theory, it seems that the method according to the present invention allows for a high improvement of the wettability and the coating adhesion of the steel sheet having a specific chemical composition. Additionally, with the method according to the present invention, it is possible to perform the alloying treatment in a reduced time. Indeed, on contrary to prior art method such as the one disclosed in JP2011153367 (FIG. 1) and as illustrated in FIG. 2, the inventors have found that the recrystallization annealing according to the present invention performed in a full Radiant Tube Furnace (RTF) wherein the heating and soaking section have the same atmosphere with DP being 18 C. and +8 C., such atmosphere comprising from 0.1 to 15% by volume of H2 allows for the production of a galvannealed steel sheet having a specific oxides repartition allowing a high wettability and having a high quality. In particular, the oxides including MnO, FeO and Mn.sub.2SiO.sub.4 are formed during the recrystallization annealing externally at the steel sheet surface and also internally allowing a high wettability and coating adhesion. Preferably, the external oxides are present in form of nodules at the sheet sheet surface. Thus, during the alloying treatment, the iron of the steel can easily diffuse towards the coating in a reduced time.

(8) If the recrystallization annealing of the above specific steel sheet is not performed according to the present invention, in particular if the heating and soaking sections do not have the same atmosphere and if the dew point is below 18 C., there is a risk to form oxides such as MnO, FeO and Mn.sub.2SiO.sub.4, such oxides being mainly or only external. Moreover, there is a risk that these oxides form a thick continuous layer at the steel sheet surface decreasing significantly the wettability. In this case, there is no interest to perform the alloying treatment in order to obtain a galvannealed steel sheet.

(9) Moreover, if the heating and soaking sections do not have the same atmosphere and if the dew point is above 8 C., there is a risk to form external oxides such as MnO and FeO and internal oxide such as Mn.sub.2SiO.sub.4. Especially, there is a risk that MnO and mainly FeO are formed in a form of a continuous layer at the steel sheet surface decreasing the wettability. In this case, there is no interest to perform the alloying treatment in order to obtain a galvannealed steel sheet.

(10) Regarding the chemical composition of the steel, the carbon amount is between 0.05 and 0.20% by weight. If the carbon content is below 0.050%, there is a risk that the tensile strength is insufficient. Furthermore, if the steel microstructure contains retained austenite, its stability which is necessary for achieving sufficient elongation, cannot be obtained. In a preferred embodiment, the carbon content is in the range between 0.05 and 0.15%.

(11) Manganese is a solid solution hardening element which contributes to obtain high tensile strength. Such effect is obtained when Mn content is at least 1.5% in weight. However, above 3.0%, Mn addition can contribute to the formation of a structure with excessively marked segregated zones which can adversely affect the welds mechanical properties. Preferably, the manganese content is in the range between 1.5 and 2.9% to achieve these effects. This makes it possible to obtain satisfactory mechanical strength without increasing the difficulty of industrial fabrication of the steel and without increasing the hardenability in the welds.

(12) Silicon must be comprised between 0.1 and 0.45%, preferably between 0.1 to 0.30% and more preferably between 0.1 to 0.25% by weight of Si to achieve the requested combination of mechanical properties and weldability: silicon reduces the carbides precipitation during the annealing after cold rolling of the sheet, due to its low solubility in cementite and due to the fact that this element increases the activity of carbon in austenite. It seems that if Si amount is above 0.45%, other oxides are formed at the steel sheet surface decreasing the wettability and the coating adhesion.

(13) Aluminum must be below or equal to 0.20%, preferably below 0.18 by weight. With respect to the stabilization of retained austenite, aluminum has an influence that is relatively similar to the one of the silicon. However, aluminum content higher than 0.20% in weight would increase the Ac3 temperature, i.e. the temperature of complete transformation into austenite in the steel during the annealing step and would therefore make the industrial process more expensive.

(14) Chromium makes it possible to delay the formation of pro-eutectoid ferrite during the cooling step after holding at the maximal temperature during the annealing cycle, making it possible to achieve higher strength level. Thus, the chromium content is between 0.10 and 0.60%, preferably between 0.10 and 0.50% by weight for reasons of cost and for preventing excessive hardening.

(15) Vanadium also plays an important role within the context of the invention. According to the present invention, the amount of V is below 0.005% and preferably 0.0001V0.005%. Preferably, V forms precipitates achieving hardening and strengthening.

(16) The steels may optionally contain elements such as P, Nb, B, Mo, Ni, Ti, S, Cu, Co, N achieving precipitation hardening.

(17) P and S are considered as a residual element resulting from the steelmaking. P can be present in an amount <0.04% by weight. S can present in an amount below or equal to 0.01% by weight.

(18) Titanium and Niobium are also elements that may optionally be used to achieve hardening and strengthening by forming precipitates. However, when the Nb amount is above 0.05% and/or Ti content is greater than 0.06%, there is a risk that an excessive precipitation may cause a reduction in toughness, which has to be avoided.

(19) The steels may also optionally contain boron in quantity comprised below or equal to 0.003%. By segregating at the grain boundary, B decreases the grain boundary energy and is thus beneficial for increasing the resistance to liquid metal embrittlement.

(20) Molybdenum in quantity below or equal to 0.2% is efficient for increasing the hardenability and stabilizing the retained austenite since this element delays the decomposition of austenite.

(21) The steel may optionally contain nickel, in quantity below or equal to 0.1% so to improve the toughness.

(22) Copper can be present with a content below or equal to 0.1% for hardening the steel by precipitation of copper metal.

(23) Preferably, the chemical composition of the steel does not include Bismuth (Bi). Indeed, without willing to be bound by any theory, it is believed that if the steel sheet comprises Bi, the wettability decreases and therefore the coating adhesion as well.

(24) Preferably, in steps B.i) and B.ii), A1 comprises between 1 and 10% by volume of H2 and more preferably, A1 comprises between 2 and 8% by volume of H2, A2 being identical to A1.

(25) Advantageously, in steps B.i) and B.ii), DP1 is between 15 C. and +5 C., and more preferably, DP1 is between 10 and +5 C., DP2 being equal to DP1.

(26) In a preferred embodiment, in step B.i), the steel sheet is heated from ambient temperature to T1 with a heating rate above 1 C. per second and for example between 2 and 5 C. per second.

(27) Preferably, in step B.i), the heating is performed during a time t1 between 1 and 500 seconds and advantageously between 1 and 300 s.

(28) Advantageously, in step B.ii), the soaking is performed during a time t2 between 1 and 500 seconds and advantageously between 1 and 300 s.

(29) Preferably, in step B.ii), T2 is equal to T1. In this case, in steps B.i) and B.ii), T1 and T2 are between 750 and 850 C., T2 being equal to T1. In another embodiment, it is possible that T2 is below or above T1 depending on the steel sheet chemical composition and microstructure. In this case, in steps B.i) and B.ii), T1 and T2 are between 750 and 850 C. independently from each other.

(30) Preferably, in step B.iii), A3 comprises from 1 to 20% by weight of H2 and more preferably, from 1 to 10% by weight of H2.

(31) Preferably, in step B.iii), DP3 is below or equal to 35 C.

(32) In a preferred embodiment, n step B.iii), the cooling is performed during a time t3 between 1 and 50 seconds.

(33) Advantageously, in step B.iii), the cooling rate is above 10 C. per second and preferably between 15 and 40 C. per second.

(34) Advantageously, in step B.iv), A4 comprises from 1 to 20% and more preferably, from 1 to 10% by weight of H2.

(35) Preferably, in step B.iv), DP4 is below or equal to 35 C.

(36) In a preferred embodiment, in step B.iv), the equalizing is performed during a time t4 between 1 and 100 seconds and for example between 20 and 60 seconds.

(37) Advantageously, in steps B.iii) and B.iv), A3 is identical to A4, DP4 being equal to DP3.

(38) Preferably, in step B.iv), T4 is equal to T3. In this case, in steps B.iii) and B.iv), T3 and T4 are between 400 and 550 C. or between 550 and 700 C., T4 being equal to T3. In another embodiment, it is possible that T4 is below or above T3 depending on the steel sheet chemical composition and microstructure. In this case, in steps B.iii) and B.iv), T3 and T4 are between 400 and 550 C. or between 550 and 700 C. independently from each other.

(39) Preferably, in steps B.i) to B.iv), the inert gas is chosen from: N2, Ar, He and Xe.

(40) Preferably in step C), the zinc-based coating comprises between 0.01 and 0.4% by weight of Al, the balance being Zn.

(41) Advantageously, in step D), T5 is between 470 and 570 C., more preferably between 470 and 530 C.

(42) Preferably, in step D), t5 is between 1 and 35 seconds and for example between 1 and 20 s.

(43) In a preferred embodiment, the alloying treatment is performed in atmosphere A5 comprising air.

(44) The invention also relates to a galvannealed steel sheet wherein the zinc coating is alloyed through diffusion of the iron from the steel sheet such that the zinc coating comprises from 5 to 15% by weight of Fe, oxides including FeO, Mn.sub.2SiO.sub.4 and MnO, the balance being zinc, the steel sheet comprising internal oxides including FeO, Mn.sub.2SiO.sub.4 and MnO in the steel sheet. Preferably, the oxides comprising FeO, Mn.sub.2SiO.sub.4 and MnO present in the zinc or aluminum coating are in a form of nodules.

(45) Preferably, the thickness of the coating is between 1 and 15 m.

(46) Preferably, the steel microstructure comprises bainite, martensite, ferrite and optionally austenite. In one preferred embodiment, the steel microstructure comprises from 1 to 45% of martensite, from 1 to 60% of bainite, the balance being austenite. In another preferred embodiment, the steel microstructure comprises from 1 to 25% of fresh martensite, from 1 to 10% of ferrite, from 35 to 95% of martensite and lower bainite and less than 10% of austenite.

(47) In a preferred embodiment, the surface of steel sheet is decarburized. Preferably, the depth of the decarburization is up to 100 m, preferably up to 80 m, from the surface steel sheet. In this case, without willing to be bound by any theory, it is believed that the steel sheet has a better resistance to LME due to the reduction of carbon amount into the steel sheet. Indeed, it seems that carbon is an element highly sensitive to liquid metal embrittlement LME. Additionally, better bendability and better crash behavior.

(48) Finally, the invention relates to the use of the galvannealed steel sheet for the manufacture of a part of an automotive vehicle.

(49) The invention will now be explained in trials carried out for information only. They are not limiting.

EXAMPLES

(50) In this example, DP steels having the following composition in weight percentage were used:

(51) TABLE-US-00001 C Mn Si Cr Al Mo Ti P S Cu Ni Nb V B N 0.072 2.52 0.255 0.30 0.15 0.1 0.017 0.013 0.001 0.015 0.021 0.025 0.004 0.0020 0.006

(52) All Trials being DP steels were annealed from ambient temperature in a full RTF furnace according to the conditions of Table 1.

(53) Then, all Trials were hot-dip coated in a zinc bath containing 0.117% of Aluminum.

(54) After the coating deposition, the trials were analyzed by naked eyes, scanning electron microscope and Auger spectroscopy. For the wettability, 0 means that the coating is continuously deposited and 1 means that the coating is not continuously deposited. When the wettability was of 0, i.e. really good, the Trials were alloyed in order to obtain a galvannealed steel sheet. When the wettability was of 1, i.e. very bad, there was no need to alloy since the quality of the coating was very bad due to the presence of a lot of unwanted oxides are present at the steel sheet surface.

(55) Results are shown in the Table 1 below.

(56) TABLE-US-00002 Presence of FeO, Mn2SiO4, MnO Coat- Alloying Oxides ing Heating section (A1) Soaking section (A2) Cooling section (A3) Equalizing (A4) treatment in the In thick- Tri- DP1 T1 % t1 DP2 T2 % t2 DP3 T3 % t3 DP4 T4 % t4 Wetta- T5 t5 coat- the ness als ( C.) ( C.) H2 (s) ( C.) ( C.) H2 (s) ( C.) ( C.) H2 (s) ( C.) ( C.) H2 (s) bility ( C.) (s) ing steel (m) 1 +18 780 5 209 +18 780 5 72 40 460 5 10 40 460 5 35 1 ND ND 2 +15 780 5 209 +15 780 5 72 40 460 5 10 40 460 5 35 1 ND ND 3 +10 780 5 209 +10 780 5 72 40 460 5 10 40 460 5 35 1 ND ND 4* +5 780 5 209 +5 780 5 72 40 460 5 10 40 460 5 35 0 470 20 yes yes 9.4 5* 0 780 5 209 0 780 5 72 40 460 5 10 40 460 5 35 0 470 28 yes yes 9.0 6* 10 780 5 209 10 780 5 72 40 460 5 10 40 460 5 35 0 470 40 yes yes 9.7 7* 15 780 5 209 15 780 5 72 40 460 5 10 40 460 5 35 0 470 40 yes yes 9.5 8 20 780 5 209 20 780 5 72 40 460 5 10 40 460 5 35 1 ND ND 9 30 780 5 209 30 780 5 72 40 460 5 10 40 460 5 35 1 ND ND 10 40 780 5 209 40 780 5 72 40 460 5 10 40 460 5 35 1 ND ND 11 50 780 5 209 50 780 5 72 50 460 5 10 50 460 5 35 0 470 76 no no 10.7 12 60 780 5 209 60 780 5 72 60 460 5 10 60 460 5 35 0 470 72 no no 10.8 *Examples according to the present invention. ND: not done.

(57) Trials 4 to 7 according to the present invention and Examples 11 and 12 show a good wettability. Nevertheless, for Trials 4 to 7, the alloying time was significantly reduced compared to Trials 11 and 12. Moreover, the surface aspect of the coating was significantly good for the Examples according to the present invention.